The invention relates to luminance and/or radiance meters and to the measurement of certain characteristics of a selected area of a luminous light source. In particular the invention is directed toward a device for measuring size and brightness uniformity of the area being measured, contrast ratio of such area, modulation transfer functions, and for microdensitometry purposes.
Various optical systems for measuring fields in luminance and radiance meters have been utilized such as described in an article entitled "Optical Systems for Defining the Viewing and Measuring Fields in Luminance/Radiance Meters" by Richard A. Walker, published in Volume 11, No. 9 of the September 1972 issue of Applied Optics. While such optical systems defined the viewing and measuring fields of a subject, scanning of the subject generally required either moving the entire photometer relative to the subject, moving the imaging lens system, or moving a part of the lens system. In one prior proposed scanning system, a fiber optic probe with a measuring aperture attached thereto was laterally moved to scan the focal plane of a lens. Such scanning system moved off the optical axis of the lens and any such off-axis measurements were subject to reduction by the fourth power of the cosine of the off-axis angle.
In another prior proposed scanning system, the measuring aperture was stationary and the objective lens was laterally moved to scan the subject field. Such scanning system was also subject to the cosine fourth law disadvantage.
Further, prior proposed means for scanning and measuring light from a subject area included devices such as cross slides, rotary tables, and motorized drives for moving an entire instrument having an optical head in a linear or rotary motion. Such instrument moving means were capable of providing precise linear of angular scanning of the subject area but included disadvantages in that the means for moving the instrument usually had several times the mass of the instrument being moved. Thus movement of the instrument resulted in difficulty in rendering the system isolated from unwanted vibrations and in precisely controlling its movement.
Image scanners mentioned above included disadvantages in that when an aperture is moved off of the optical axis, the measurement of luminance or radiant power is subject to the cosine-4th Law of Image Illumination. Another disadvantage is that the dimensions measured in the subject area depend upon the objective lens magnification which changes with the distance from the object and the off-axis angle.